1. Field of the Invention
The present invention relates to a liquid crystal display device having a touch panel and more particularly to a noise suppression circuit for the touch panel.
2. Description of the Background Art
Recent electronic equipments have used liquid crystal display devices have been used with a touch panel in order to achieve cost reduction, lightness and compactness by a reduction in a number of buttons and an increase in operability of the display panel. However, a problem arises due to touch panel malfunction caused by EMI (Electro Magnetic Interference). For example, when a picture is drawn using touch pen contact on the liquid crystal display panel, if EMI noises are generated, a correct picture tracing the movement of the touch pen may not be drawn because a signal that shows contact of the touch pen breaks up during the drawing.
The above problems are caused by following noises:    (1) a noise generated in the liquid crystal display panel due to abrupt changes of voltages of signal lines in the liquid crystal display panel.    (2) a high frequency noise due to an abrupt rise or fall of a counter electrode voltage Vcom.    (3) a noise due to an abrupt rise or fall of an analog pixel voltage of Vsig.    (4) a noise generated by an interaction of noises contained in the counter electrode voltage Vcom and a liquid crystal line voltage Vcs.
One proposal to prevent the noises generated in the LCD panel or a LCD driver circuit from reaching the touch panel is, for example, to arrange a transparent shield plate to shield the noises between the liquid crystal display panel and the touch panel. However, in such an approach, sufficient thickness is required to achieve a suitable shielding effect. On the other hand, as the shield plate becomes thicker, insufficient light transmission is obtained, which results in darkness of the panel. Moreover, if a shield plate is used, the cost of parts composing the display is increased and manufacturing processes become complicated. Accordingly, use of a shield plate is problematic.
Japanese patent application No. 1997-352837 proposes a noise elimination circuit in which noises generated in a LCD panel are discarded from input data by the touch panel. However, the technology does not eliminate the origin of the noise, therefore, it is difficult to achieve a fundamental solution to solve the problem with high reliability.